In the prior art, a permanent magnet field type direct current motor is prevailingly employed as a servomotor. A tendency is recognized, however, that a permanent magnet revolving field type synchronous motor is employed as a servomotor. This is because a synchronous motor inherently has various advantages and features pointed out below. (a) Since a synchronous motor is a brushless motor, it is free from potential disturbance for wireless communication systems. (b) Since no commutator is required, it is free from the problem of wear. (c) Since the rotor of a synchronous motor is durable, quick acceleration and/or retardation are or is allowed for a synchronous motor, and it is relatively easy to adjust the dynamic balance of the rotor thereof. In addition to the foregoing advantages and features inherent to a synchronous motor, an additional advantage briefed below is a parameter to enhance the employment of a synchronous motor as a servomotor. Namely, a control system briefed below can be applied to a synchronous motor. (a) Firstly, a means for detection of the angular position of a rotary object, such as a resolver, a pulse coder, etc, is employed for monitoring the angular position of a field pole. (b) Secondly, following the angular position detected above, a poly-phase alternating current voltage which generates a rotating field which is in the leading phase by e.g. .pi./2 in terms of electric angle ahead of the field pole, is generated. As a result, the frequency and phase of the poly-phase alternating current voltage follow the rotation of the rotor. (c) Thirdly, the amplitude of the poly-phase alternating current is selected to meet the amount of torque required. (d) Fourthly, when the poly-phase alternating current is supplied to a synchronous motor, the synchronous motor continues rotation with a fixed amount of internal phase angle of e.g. .pi./2 in terms of electric angle.
A slightly detailed description will be presented below for the foregoing control system applicable to a synchronous motor.
The torque T of a synchronous motor is given by the following formula: ##EQU1## wherein, V represents a voltage applied to the stator,
E represents an induced voltage, PA1 X represents the inductance of the stator, and PA1 .delta. represents an internal phase angle.
Provided the amount of internal phase angle is kept at .pi./2, the amount of sin.delta. is kept at 1 (one). On the other hand, since the amount of the stator inductance X depends on the frequency of the applied voltage, and since the amount of the induced voltage depends on the rotation speed of the rotor, the influence of the stator inductance X and the induced voltage E offsets each other. As a result, the torque T is determined depending on the amount of the applied voltage or the amount of the stator current, resultantly simplifying the torque regulation.
Supposing the angular position of a rotor specifically of a field pole is continually monitored, it is possible to generate an alternating current signal Acos .omega.t which is kept in the same phase with the rotation of a rotor. This in-phase signal can be readily converted to other alternating current signals including (a) Asin .omega.t which is in the leading phase by .pi./2 with respect to the rotor, (b) ##EQU2## which is in the lagging phase by 2/3.pi. with respect to Asin .omega.t, and (c) ##EQU3## which is in the lagging phase by 2/3.pi. with respect to ##EQU4##
The foregoing three alternating current signals can be employed as a set of trigger signals to produce a three-phase alternating current voltage which is generated by means e.g. of an inverter and which generates a rotating field which is in the leading phase by .pi./2 with respect to the rotor. Therefore, application of this three-phase alternating current voltage to a synchronous motor causes the motor to continue operation maintaining the fixed amount of internal phase angle of .pi./2 for the entire operation period, regardless of variation of the rotating speed. Further, it is possible to maintain a rotor at an arbitrary angular position, if the frequency of the three-phase alternating current voltage is decreased to and maintained at zero.
As a result, combined application of an ordinary speed control system which determines a desirable amount of current or torque and the foregoing control system, enables the motor to be controlled in the digital manner or precisely by the angle corresponding to the cumulative frequency applied to the stator. In other words, the synchronous motor receives a poly-phase alternating current voltage of which the amplitude corresponds to the torque which is determined by the ordinary speed control system for the purpose of causing the motor to follow the positional and/or speed reference signal and of which the phases are sufficient to keep the internal phase angle at a fixed amount, e.g. .pi./2, in terms of electric angle during the entire operation period wherein the motor may increase and/or decrease the rotating speed following an external command.
In order to improve the accuracy of the foregoing control system, it is essential to accurately detect the absolute angular position of each of the permanent magnet field poles which are components of a rotor. Namely, an accurate means for detection of an absolute angular position of a rotary object is essential. Supposing this means is realized employing a pulse coder based on the pure binary code system, an extremely large number of bits is required for the pulse coder. For example, 13 bits are required to split 360 degrees into 8,192 angular portions, and 14 bits are required to split 360 degrees into 16,384 angular portions. It is not easy to produce a pulse coder having such a large number of bits. Even if a pulse coder having a large number of bits can be produced, the pulse coder may be prone to malfunction, resulting in lowered reliability.